It has been known for many years that in organized media (e.g. detergent micelles) applied magnetic fields cause an increase in average free radical concentration and lengthen the lifetime of free radicals. If these effects occur in vivo then this may explain some of the reported biological effects of EMF. The aim of this project is to determine whether cellular components such as membranes, nucleic acids proteins can act as biological reaction vessels in which externally applied magnetic fields or magnetic fields generated by magnetite particles found in magnetosomes, can modulate the activity of free radicals. We have previously shown that magnetite (Fe3O4) encapsulated in polystyrene microspheres (0.002%) dramatically increased the photohemolysis of human erythrocytes by ketoprofen. We have now found that the same particles decrease hemolysis observed in the dark after erythrocytes had been irradiated (>400 nm) for 20 min. in the presence of 2 micromolar merocyanine 540 (MC540). Furthermore, the same decrease in hemolysis was observed even when the particles were removed immediately after the light source had been turned off. Non-magnetic particles were without effect. The magnetic particles also decreased MC540/light (>400 nm) induced damage to L1210 cells as measured by the MTS assay. Although epidemiological studies suggest that there is a weak relationship between exposure to electromagnetic fields (EMF) and cancer there are at present few plausible molecular mechanisms to explain this effect. It is well known that in organized media (e.g. detergent micelles) applied magnetic fields cause an increase in average free radical concentration and lengthen the lifetime of free radicals. The known involvement of free radicals in the etiology of cancer and other diseases raises the possibility that EMF may somehow act by altering the lifetime and/or concentration of free radicals. Our findings suggest that magnetic fields, either externally applied or present in the vicinity of magnetite particles in magnetosomes, may modulate the lifetime of radicals in vivo - epidemilogical study, erythrocyte, etiology, cancer, ketoprofen